Hi Simon
The element is listed by Bird as:
Product Description: C1 Series, 100-250 MHz Standard Elements, 100µA on
http://www.btgstore.com/10mhzplelc1s.html
Now while the more common inserts are for 30uA it may be possible to re calibrate the 100uA one you have to work with a 30uA meter.
Inserts have an internal pot for calibration purposes. This is under the disc on the front of the insert. This disk can be removed by placing this end of the insert in a shallow bath of petrol to loosen the adhesive used to fix the disk to the insert body. I did this to one that was out of calibration. I fixed the disk back on using PVA Wood Glue (I will need to use water to remove the disk next time).
You will need to check the readings at a few meter deflection positions to make sure the readings are tracking correctly. Now if the pot adjustment does not provide the desired results the next (and better) option is to add a resistor inside the insert in parallel with the meter contacts. This would be so that with full scale deflection, the meter takes 30uA and the resistor 70uA. You will need to determine what value resistor you need to use.
The white cap on the insert is held in place by a screw under the disk.
Hope this helps
Igor
VK6ZFG